“To walk away from this position and this power is not an easy thing,” he said. “But I think it’s the right thing. And I have two years left. I’m not passing the torch sitting down — it’s a running relay. I intend to be very active over the next two years.”

Harkin said in a statement Saturday that two primary factors drove his decision to retire: He's served in the House and Senate for 40 years and would be 75 at the end of his current term, and he had made a promise to his wife that "we’re going to live together in a way we’ve often talked about — before it gets too late."

He told the Register he made the decision with his wife Ruth on Wednesday. "I'm going to learn to dance," he said.

Harkin added in his statement that his retirement from the Senate did not mean would be disappearing from the public eye. "I don’t by any means plan to retire completely from public life at the end of this Congress," he said.

Harkin’s retirement makes Iowa a ripe target for Republicans
as they try to flip at least six seats to win back the Senate.

While Harkin was likely in for a contested race if he had
run for reelection, he had $2.7 million in the bank already, according to
Federal Election Commission records. He had a February fundraiser planned at $1,500-a-plate with pop superstar Lady Gaga, according to the Register.

Rep. Steve King (R-Iowa) did
not rule out a 2014 Senate run in November, regardless of whether Harkin would have run.

"I have a consistent record. I think people know
me," he said, arguing his views wouldn't be "an impediment at
all."

Ten-term Rep. Tom Latham (R-Iowa) is the other Iowa
Republican in the House. A primary between King and Latham could follow the tea party vs. establishment narrative that has played out over
the past two cycles in Republican primaries.

Latham didn't give any hint of his intentions in a statement from his spokesman on Harkin's retirement.

"Congressman Latham respects Senator Harkin's decision," Latham spokesman James Carstensen said. "He looks forward to continuing to working with him and the rest of the Iowa delegation for the best interests of the people of Iowa over the next two years."

On the Democratic side, possible candidates include Rep.
Bruce Braley, who has also talked about running for governor, Agriculture
Secretary Tom Vilsack and his wife, Christie Vilsack, who lost to King in a 2012
run for Congress.

Obama campaign strategist David Axelrod predicted on Twitter Saturday that the race could wind up between Braley and King in a battle of House members.

“Harkin's exit could set up defining battle between 2 House members: Bruce Braley, a charismatic D; and Steve King, a hero of the far Right,” Axelrod tweeted.

President Obama said he and first lady Michelle Obama joined Iowans in thanking Harkin for his service.

“By the time Senator Harkin finishes his fifth term, he will have represented the people of Iowa in the United States Congress for an incredible 40 years. He has served in the U.S. Senate longer than any Democrat in Iowa's history,” Obama said in a statement.

“During his tenure, he has fought passionately to improve quality of life for Americans with disabilities and their families, to reform our education system and ensure that every American has access to affordable health care,” the president added.

"I appreciate that Senator Harkin has made this decision so early in the cycle, giving us ample time to recruit a strong Democratic candidate for this seat," Sen. Michael Bennet (D-Colo), chair of the Democratic Senatorial Campaign Committee, said in a statement. "Iowa has a strong record of electing great Democrats, and I'm confident that we will elect a new Democratic senator next November."

Harkin, who was one of the few Democrats to vote against the "fiscal-cliff" deal, passed on the chance to chair the powerful Senate Appropriations
Committee in November. He opted instead to stay at the head of the Health,
Education, Labor and Pensions Committee.

He is the second Senate Democrat to opt to retire rather
than run for another term, as Sen. Jay Rockefeller (D-W.Va.) announced
earlier this month that he would not run in 2014.

Republican Sen. Saxby Chambliss (Ga.) also said Friday that he
would retire at the end of his term.